1. Field of the Invention
The present invention relates to an optical recording/reproducing apparatus for optically recording a signal on a recording medium and for optically reproducing the recorded signal, by using a light beam from a light source such as a laser. More particularly, the present invention relates to a focus control method and a focus control apparatus both for a focusing pull for controlling the converged state of a light beam with which the recording medium is irradiated and maintaining a prescribed converged state.
2. Description of the Related Art
An exemplary conventional focus control apparatus is an optical recording/reproducing apparatus in which a signal is recorded on or reproduced from a disk-like recording medium by converging and radiating a light beam generated from a light source such as a semiconductor laser on the recording medium which rotates at a predetermined rotation frequency. On the recording medium, minute tracks each having a width of 0.6 .mu.m are provided at a pitch of 1.5 .mu.m in a spiral or concentric manner. In order to record a signal on the track or reproduce the signal recorded on the track, the optical recording/reproducing apparatus performs a focus control so that the light beam with which the recording medium is irradiated is in a prescribed converged state.
FIG. 5 is a block diagram showing a schematic construction of such a conventional recording/reproducing apparatus including a focus control apparatus. Hereinafter, the conventional focus control apparatus will be described with reference to FIG. 5.
As shown in FIG. 5, the conventional recording/reproducing apparatus includes an optical system for irradiating a disk 7 as a recording medium with a light beam 8, and a disk motor 6 for rotating the disk 7 at a predetermined rotation frequency. The optical system includes a light source 1 such as a semiconductor laser, a coupling lens 2, a polarization beam splitter 3, a polarizing plate 4, and an objective lens 5. The light beam 8 generated from the light source 1 is collimated by the coupling lens 2. The collimated light is then reflected from the polarization beam splitter 3 and passes through the polarizing plate 4. Then, the light is converged by the objective lens 5, and incident on the disk 7 which is rotated by the disk motor 6.
The recording/reproducing apparatus further includes a converging lens 9 and a slit mirror 10 as elements for receiving the light reflected from the disk 7. The light reflected from the disk 7 passes through the objective lens 5, the polarizing plate 4, and the polarization beam splitter 3, and split into light beams 11 and 15 in two directions by the split mirror 10 via the converging lens 9. The light beams 11 and 15 are input into a focus control apparatus and a tracking control apparatus, respectively.
The focus control apparatus includes a 2-section photodetector 12, preamplifiers 13A and 13B, a differential amplifier 14, a phase compensation circuit 18, a linear motor 19, a switch 33, a driving circuit 35, a focus control element (a focus actuator) 36, a logic circuit 40, a comparator 41, and a triangular-wave generator 42. The photodetector 12 has two photodetecting portions A and B. Signals output from the respective photodetecting portions A and B are amplified by the preamplifiers 13A and 13B, and then input into the differential amplifier 14. By means of the converging lens 9 and the split mirror 10, a knife edge detecting method can be realized, so that the output signal of the differential amplifier 14 functions as a focus error signal.
As to the focus error signal, the phase in the focus control system is compensated for by the phase compensation circuit 18. Then, the focus error signal is input into the driving circuit 35 via the switch 33 for closing a loop of the focus control system. When the focus control system is closed by the switch 33, the driving circuit 35 amplifies the power of the focus error signal from the phase compensation circuit 18, and outputs the amplified signal to the focus control element 36. With the above-described construction, the focus control element 36 is driven so that the light beam converged on the disk is in the prescribed converged state, when the loop of the focus control system is closed. To the switch 33, the output signal from the triangular-wave generator 42 is also input. The focus error signal is also input into the logic circuit 40 via the comparator 41. The logic circuit 40 controls the open/close state of the switch 33.
The linear motor 19 moves the objective lens 5, the focus control element 36, the polarization beam splitter 3, and the like in a direction across the tracks on the disk 7. The linear motor 19 is usually activated when the converged point of the light beam is to be moved onto a predetermined track.
On the other hand, the other light beam 15 which is split by the split mirror 10 is input into the 2-section photodetector 16 of the tracking control apparatus. The photodetector 16 has two photodetecting portions C and D. A difference output signal between the output signals from the photodetecting portions C and D functions as a tracking error signal for controlling the light beam on the disk 7 so as to properly scan the tracks. The tracking control is not directly related to the invention, so that the detailed description of the tracking control is omitted.
In the recording/reproducing apparatus having such a focus control apparatus, the focus control is performed as follows.
First, the disk 7 is rotated by the disk motor 6. When the rotation of the disk 7 attains a predetermined rotation frequency, the switch 33 is switched to the side of the triangular-wave generator 42. Thus, the focus control element 36 is driven by the signal having the triangular waveform from the triangular-wave generator 42. Accordingly, the objective lens 5 is moved downward or upward in a direction perpendicular to the recording face of the disk 7. Therefore, the converged point of the light beam on the disk 7 is moved downward or upward. An S-curved focus error signal (hereinafter referred to as an S-curved signal) appearing when the converged point of the light beam passes through the recording face is detected by the comparator 41. Based on the detection of the S-curved signal, the logical circuit 40 can tell whether the converged point of the light beam exists in the vicinity of the recording face or not. If the converged point exists in the vicinity of the, recording face, the switch 33 is switched to the side of the phase compensation circuit 18. In this way, the focus control loop is closed, so that the converged point of the light beam is located at a desired optimum position by the focus control (i.e., focusing pull).
The operation of the focusing pull is described with reference to FIGS. 6, 7, and 8. FIG. 6 shows the waveforms of the S-curved signals appearing in the objective lens driving signal and the focus error signal during the focusing pull. FIG. 7 shows a waveform illustrating a relationship between a focus on level end the S-curved signals in a protect film (which serves as a disk surface) and a recording film of the disk 7 which appear in the focus error signal when the objective lens 5 is moved closer to or away from the disk 7. The term "focus on level" means a level at which the focusing pull is attained. FIG. 8 shows a schematic flowchart illustrating a basic focusing pull procedure in the focus control apparatus.
As shown in FIG. 8, when the power of the recording/reproducing apparatus is turned on, the disk motor 6 is activated in step S21, so that the disk 7 is rotated. When the rotation of the disk 7 attains a predetermined rotation frequency, the light source 1 is turned on in step S22, so that, for example, the semi-conductor laser emits light. Then, in step S23, the linear motor 19 starts to operate, so that the objective lens 5 is moved closer to the center of the disk 7, i.e., to the inner side of the disk 7. After the above-described initial operation is completed, the focusing pull operation is started.
First, as shown in (a) of FIG. 6, in response to the output signal from the triangular-wave generator 42, the objective lens 5 is moved downward, i.e., moved away from the disk 7, in step S24. Then, in step S25, the objective lens 5 is moved upward, i.e., moved closer to the disk 7. During the repetition of the up and down movement of the objective lens 5, it is detected in step S26 that the S-curved signal reaches a predetermined focus on level. After the predetermined focus on level is reached, the switch 33 is switched to the side of the phase compensation circuit 18 by the logic circuit 40. In step S27, the up and down movement of the objective lens 5 is stopped, and in step S28, the focus control is turned on. Thus, the pull operation is finished, and the focus control is started.
The detection level (focus on level) of the comparator 41 for pulling the focus is defined by the amplitudes of the S-curved signals which are respectively output due to the reflection from the recording film and the reflection from the protect film of the disk 7. As shown in FIG. 7, the focus on level is set to be greater than the peak of the S-curved signal of the protect film, and set to fall within the linear interval between the peak of the S-curved signal of the recording film and zero.
According to the above-described method, the conventional focus control apparatus realizes the focusing pull operation of the focus control. However, according to the conventional method, if there is only a small difference or no difference between the reflectivity of the protect film and the reflectivity of the recording film of the disk 7, it is difficult to set the detection level of the comparator 41. Depending on the variety of disks 7 and/or the level of noise, in some cases, the S-curved signal of the protect film may be erroneously detected as the S-curved signal of the recording film, so that the switch 33 is closed. In such a case, the protect film portion is not in the linear interval of the focus (pull range), so that the focus cannot be pulled. This may often cause a failure of activation of the focus control apparatus.
Japanese Laid-Open Patent Publication Nos. 64-89027 and 57-150147 propose methods for solving such a problem.
In the method proposed in Japanese Laid-Open Patent Publication No. 64-89027, the objective lens is initially moved closest to the disk, and the converged point of the light beam is set beyond the recording film. The detection of the recording film is surely performed by using the S-curved signal first appearing when the objective lens is moved away from the initial position. In the method proposed in Japanese Laid-Open Patent Publication No. 57-150147, the number of appearing times of the S-curved signal is counted. Thus, when the objective lens is moved closer to the disk, it is detected that the objective lens reaches a position at which the converged point of the light beam is just on the recording film, so that the focus is pulled.
By these methods, the focus can be surely pulled in the S-curved signal portion of the recording film. However, it is necessary to appropriately adjust the signal level of the focus error signal in order to prevent any malfunction due to the saturation and level reduction of the focus error signal actually caused by the variation in reflectivity of the disk. If such an adjusting operation is introduced during the activation of the focus control apparatus, as shown in the waveforms in FIG. 9 and the flowchart of FIG. 10, in general, the objective lens should be moved close to (step S39) and away from (steps S35 and S43) the disk. Thus, the measurement of the amplitude of the S-curved signal (steps 836, 840, and 844 ) and the measurement and adjustment of the focus on level by the comparator based on the measurement (steps S37, S38, S41, and S42) are required.
The speed for moving the objective lens closer to or away from the recording medium for a focusing pull is about 3 mm/s, and the required focusing pull time is about 500 ms to 1 sec. in view of the time for moving the objective lens in the radial direction by a linear motor and the like. The activation time of the recording/reproducing apparatus is about 5 sec., so that the ratio of the time for the focusing pull to the total activation time is high. In addition, in order to secure the stability of the pull, if the S-curved signal measuring operation and the changing operation for the input gain are introduced for the focusing pull, the activation time becomes too long.
In a disk (specifically, CD and LD), the magnitude of variation of the face in a horizontal level (hereinafter referred to as a face variation magnitude) is large. In some CDs, the magnitude may be .+-.500 .mu.m at the peripheral portion (at a standard rotation frequency, the acceleration is 10 m/s.sup.2). With a large face variation magnitude, the relative speed when the objective lens is moved closer to and away from the disk is varied. Thus, in some cases, the time interval in which the pull operation can be performed may be very short, so that it is difficult to perform the focusing pull operation. That is, when the focus control operation is started after the S-curved signal is detected, the converged point of the light beam is already positioned outside the pull range, so that the focusing pull cannot be performed due to the nonlinear operation. Even if the focusing pull can be performed, the over shoot of the focus error signal immediately after the pull is large, and hence the control is unstable. Moreover, if the acceleration due to the face variation magnitude of the disk is large or there occurs external vibration shock, the focusing pull may often be failed and the focus control apparatus cannot be activated. Thus, the reliability of the apparatus is unacceptable.